Engines may be operated with boosted aircharge provided via a turbocharger wherein an intake compressor is driven by an exhaust turbine. However, placing a turbine in an exhaust system can increase engine cold-start emissions due to the turbine acting as a heat sink. In particular, engine exhaust heat during the engine cold-start may be absorbed at the turbine, lowering the amount of exhaust heat that is received at a downstream exhaust catalyst. As such, this delays catalyst light-off. Consequently, spark retard may be required in order to activate the exhaust catalyst. However, the fuel penalty associated with the spark retard usage may offset or even outweigh the fuel economy benefit of the boosted engine operation.
Accordingly, various approaches have been developed to expedite the attainment of a catalyst light-off temperature during cold-start conditions in a boosted engine. One example approach, shown by Andrews in U.S. Pat. No. 8,234,865 involves routing exhaust gas towards an exhaust tailpipe via a passage that bypasses the exhaust turbine during cold-start conditions. A passive, thermatically operated valve is used to regulate the flow of exhaust through the passage, the valve opening during low-temperature conditions (such as during cold-start). The thermatically operated valve comprises a bi-metallic element which distorts based on temperature thereby regulating the opening of the valve. By circumventing the turbine, exhaust heat may be directly delivered to the exhaust catalyst.
However, the inventors herein have recognized potential issues with such systems. As one example, after catalyst light-off, the temperature of the unobstructed exhaust reaching the catalyst may be higher than desired. In particular, owing to a coating on the catalyst surface (such as on the surface of an exhaust oxidation catalyst or three-way catalyst), the catalyst may have higher conversion efficiencies at lower exhaust temperatures. As a result, the higher than desired temperature of exhaust reaching the catalyst may result in reduced catalyst functionality.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example method includes flowing exhaust gas from an engine first through an emission control device and then through a turbine to rotate the turbine in a reverse direction, the rotation of the turbine in the reverse direction generating intake manifold vacuum for a vacuum consumer via a compressor coupled to the turbine.
In this way, the exhaust gas may be first routed through the emission control device before being routed to the turbine, thus expediting catalyst warm-up. However, the exhaust gas is still routed to the turbine, but in a manner that causes reverse rotation of the turbine. In doing so, intake manifold vacuum may be generated for a vacuum consumer, such as a brake booster. By operating the turbine and compressor in reverse to generate intake manifold vacuum, an intake throttle may be dispensed with, thus lowering system cost and controls complexity.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.